The present invention relates to offset printing presses and, particularly, to the electronic control of ink supplied by such presses.
Web offset printing presses have gained widespread acceptance by metropolitan daily as well as weekly newspapers. Such presses produce a quality black and white or color product at very high speeds. To maintain image quality, a number of printing functions must be controlled very precisely as the press is operating. These include the control of press speed, the control of color register, the control of ink flow and the control of dampening water.
In all printing processes there must be some way to separate the image area from the non-image area. This is done in letterpress printing by raising the image area above the non-image area and is termed "relief printing". The ink roller only touches the high part of the plate, which in turn, touches the paper to transfer the ink. In offset lithography, however, the separation is achieved chemically. The lithographic plate has a flat surface and the image area is made grease-receptive so that it will accept ink, and the non-image area is made water-receptive so it will repel ink when wet.
In a web offset printing press the lithographic plate is mounted to a rotating plate cylinder. The ink is injected onto an ink pickup roller and from there it is conveyed through a series of transfer rollers which spread the ink uniformly along their length and transfer the ink to the image areas of the rotating plate. Similarly, dampening water is applied to a fountain roller and is conveyed through one or more transfer rollers to the non-image areas of the rotating plate cylinder. The plate cylinder rotates in contact with a blanket cylinder which transfers the ink image from the plate cylinder to the moving paper web.
It is readily apparent that the amount of ink and dampening water supplied to the plate cylinder is directly proportional to the press speed. At higher press speeds the plate cylinder and blanket cylinder transfer ink and water to the paper web at a higher rate, and the inking and dampening systems must, therefore, supply more ink and water. It is also well known that this relationship is not linear and that the rate at which ink and dampening water is applied follows a complex rate curve which is unique to each press and may be unique to each run on a press. Not so apparent is the fact that the ink and water may be applied nonuniformly across the width of the ink pickup roller and the fountain roller in order to achieve uniform printing quality along the width of the web. If this is not done, there may be significant changes in the quality of the printed images across the width of the moving web.
Ink is normally supplied to web fed printing presses from an ink fountain onto a fountain roller that is in operative contract with a ductor roller which forwards the ink to the remaining ink train rollers. The feeding of ink into a press, particularly one using the lithographic process, is a demanding operation, yet one which is vital to successful printing. The oil base inks which are used in offset lithography are very viscous, in many cases being more plastic than fluid. This physical property of these inks is one reason why ink is supplied to the plate roller through a train, i.e., so that the viscous nature can be reduced and a uniform film of ink presented to the plate
The application of ink to the fountain roller has been commonly regulated by means of a blade which forms one wall of the ink fountain. The free end of this blade is adjustable by means of a manually or motor operated adjusting screw. This type of system is generally referred to as being a keyed inker and it is capable of controlling the amount of ink that is presented to each column of print across the width of the plate roll.
A more recent development is that of supplying ink to an ink rail by means of individual gear pumps that are mounted directly on the rail; one for each column of print. This type of ink supply which is shown in U.S. Pat. No. 4,281,597, regulates the amount of ink supplied to each column by varying the speed of the drive motor operating the gear pump associated with each print column. Adjustment in the speed of the motors can be performed either manually or automatically. Additional types of keyed inkers where generally a pump or a piston is used to deliver the ink through suitable dispensing nozzles onto the ink roller are disclosed in U.S. Pat. Nos. 2,981,182; 2,081,906; 3,207,070 and 2,130,659.